d
\(\frac{{{R_1}}}{{{R_2}}} = \frac{{(1 + \alpha \,{t_1})}}{{(1 + \alpha \,{t_2})}}\,\, \Rightarrow \,\)\(\frac{{20}}{{60}} = \frac{{1 + 20\alpha }}{{1 + 500\alpha }}\,\,\,\, \Rightarrow \,\,\,\,\alpha = \frac{1}{{220}}\)

\(\Rightarrow \)  \(\frac{{20}}{{25}} = \frac{{\left( {1 + \frac{1}{{220}} \times 20} \right)}}{{\left( {1 + \frac{1}{{220}} \times t} \right)}}\) \(\Rightarrow \) \(t\) = \(80\,^oC\)